Fix `Model::findMultipleByIds()` (see #6331)
Remove a redundant variable declaration in the `Form` class
Add the `system/docs/LIMITATIONS.md` file
Add the "maintenance mode" (see #4561)
Fix the Ajax "reloadPagetree" and "reloadFiletree" commands

The "load_callback" expects the current value as argument, not the new value. The attributes array was missing the active record, which is available upon regular requests but has not been upon Ajax requests.
Fix a method name in the `UPGRADE.md` file
Remove a redundant comma in the `UPGRADE.md` file
Document the `FilesModel::findByPk()` together with `$this->singleSRC` problem
Add the "getAttributesFromDca" hook (see #6340)
Tweak the new maintenance page a little